Columnar dendritic solidification of TiAl under diffusive and hypergravity conditions investigated by phase-field simulations

Abstract

Based on 2D phase-field simulations including fluid flow driven by natural convection, columnar dendritic growth of the β-solidifying Ti-48 at%Al alloy is characterised for different gravity levels ranging from 0 to ±15g. Depending on the direction of the gravity g with respect to the growth direction, different flow regimes emerge which show stable or unstable dendritic growth dynamics. When gravity and growth directions are parallel, the dendrite tips experience downward melt flow and individual dendrites grow in a stable manner with a rather small modification of the operating state. When gravity and growth directions are antiparallel, the impact on the operating state is larger. Eventually, at higher gravity levels the upward melt flow around the dendrite tips "destabilises" the dendritic morphology resulting in tip splitting, branching and local changes in the apparent dendrite growth direction which is an alternative mechanism for the adjustment of the primary dendrite arm spacing in addition to tertiary arm formation.

Mesoscopic modeling of equiaxed and columnar solidification microstructures under forced flow and buoyancy-driven flow in hypergravity: Envelope versus phase-field model

“…Naturally, a perspective of this work is to extend the mesoscopic simulations of columnar growth in hypergravity to larger domains and to 3D. The geometry and the dimension of the problem play a critical role on the flow characteristics [7,63] especially on its velocity. The smaller computational cost than for phase field allows 3D simulations coupled with flow to be done at the scale of the experiments without using a supercomputer.…”

“…Gravity drives the solutal natural convection in the liquid around the growing dendrites. Depending on the gravity direction with respect to the growth direction, very different flow regimes form and result in a stable or unstable dendritic growth [7]. When gravity and growth vector direction are aligned, the macroscopic density gradient in the mushy zone is parallel to gravity and is therefore hydrodynamically stable.…”

“…A detailed study of the physical phenomena involved in the change of the columnar growth pattern in a Ti-48 at.%Al alloy has already been performed experimentally and by phase-field simulations in Ref. [7].…”

“…Recently, Viardin et al [7] investigated the influence of hypergravity on the columnar dendritic microstructure during directional solidification of a Ti-48 at.%Al using phase-field simulations. They have shown that the direction and the level of hypergravity triggers changes of the primary dendrite arm spacing (PDAS) and of the grain morphology.…”

“…Quantitative numerical simulations are indispensable as a complementary tool for the analysis of post mortem characterizations available from the hypergravity experiments [3,8]. The remarkable power of phase-field models for the detailed description of the interaction between solidification microstructures and flow [7,[10][11][12][13][14][15] is impaired by their high computational cost. Sophisticated numerical and program-ming techniques [16] and massive supercomputing resources are necessary to be able to run simulations on domains that are large enough to capture the scales of the flow structure.…”